Measurements of Flow in Boundary Layer Ingesting Serpentine Inlets

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Virginia Tech

Highly integrated airframe-propulsion systems featuring ingestion of the airframe boundary layer offer reduced noise, emissions, and fuel consumption. Embedded engine systems are envisioned which require boundary layer ingesting (BLI) serpentine inlets to provide the needed air ow to the engine. These inlets produce distorted flow profiles that can cause aeromechanical, stability, and performance changes in embedded engines. Proper design of embedded engine systems requires understanding of the underlying fluid dynamics that occur within serpentine inlets.

A serpentine inlet was tested in a specially designed wind tunnel that simulated boundary layer ingestion in a full-scale realistic environment. The measured total pressure proles at the inlet and exit planes of the duct, and the static pressure distributions along the walls provided useful data related to the flow in BLI serpentine inlet systems. A bleed ow control system was tested that utilized no more than 2% of the total inlet ow. Two bleed slots were employed, one near the first bend of the S-duct and one near second. The bleed system successfully reduced inlet distortions by as much as 30%, implying improvements in stall margin and engine performance.

Analysis of the wake shape entering the S-duct showed that the airframe and inlet duct are both important components of a wake-ingesting inlet/diffusion system. Shape effects and static pressure distributions determined flow transport within the serpentine inlet. Flow separation within the S-duct increased distortion at the engine inlet plane. Discussion of airframe/inlet/engine compatibility demonstrates that embedded engine systems require multi-disciplinary collaborative design efforts. An included fundamental analysis provides performance estimates and design guidelines. The ideal airframe performance improvement associated with wake-ingestion is estimated.

boundary layer ingestion, wake ingestion, distortion, flow control, diffuser, inlet